The present invention relates to an ultrasonic examination apparatus for transmitting and receiving an ultrasonic wave toward and from a body to be examined in order to examine the inside of a body using the fact that the acoustic characteristic of the received ultrasonic wave is varied in accordance with the propagation characteristic within the body to be examined.
Known as a system to obtain acoustic information within a body to be examined is an ultrasonic diagnostic system which is principally of the pulse reflection type that is generally arranged to transmit an ultrasonic wave into an organism (body to be examined) and obtain the acoustic information within the organism on the basis of the echo wave from the inside of the organism. The pulse reflection system generally obtain a cross-sectional image of the body by two-dimensionally collecting and indicating information within the organism obtainable on the basis of the magnitude of the reflection echo, i.e., amplitude value and propagation time of the ultrasonic wave, from boundary surfaces with different acoustic impedances. Recently, the ultrasonic diagnostic apparatus, in addition to judgement of the configuration of the organism tissue, is further required to obtain information relating to the quality thereof. Such information relating to the quality can be obtained, for example, by measuring the degree of attenuation, the acoustic velocity, the acoustic non-linear parameter and/or the like which are inherent to various internal organs within the organism. In the case of measurement of the non-linear parameter B/A, the following relational expression may be used basically. EQU .DELTA.C=(1+B/2A).multidot.v (1)
where v is the particle velocity of the acoustic wave, B/A is the acoustic non-linear parameter of a medium and .DELTA.C represents the variation of the acoustic velocity based upon the non-linear effect.
It will be understood from the equation (1) that the acoustic velocity is increased when the direction of the particle velocity of the acoustic wave is coincident with the advancing direction of the acoustic wave and the acoustic velocity is reduced when reversed thereto, resulting in distortion of the waveform of the acoustic wave. For measuring this distortion of the waveform, conventional apparatus have been arranged to transmit an ultrasonic wave into a body to be examined, analyze the distortion of the obtained reception signal and perform comparison of the levels of the higher harmonic components for estimation of the non-linear parameter.
There is a problem which arises with this type of distortion-measuring apparatus, however, in that the error is essentially great in the case of obtaining the non-linear parameter from the higher harmonic component of the pulse-echo reception signal for a medium such as an organism in which the attenuation of the ultrasonic wave is greater and the attenuation characteristic depends upon the frequency, thereby resulting in occurrence of frequency dispersion of the acoustic velocity.
Furthermore, known as another example to obtain the information within the body using an ultrasonic wave is a non-destructive examination system which is arranged to transmit an ultrasonic wave into a structure and to obtain information in the inside of the structure on the basis of the reflection wave from the inside thereof. This is similarly arranged so as to obtain a cross-sectional image of the body by two-dimensionally collecting and indicating information within the structure obtainable on the basis of the magnitude of the reflection echo, i.e., amplitude value and propagation time of the ultrasonic wave, from boundary surfaces with different acoustic impedances. Recently, it is also required that the non-destructive examination system, in addition to the examination of defects and so on based upon the configuration of the inside of the structure, obtains information relating to the quality of the materials making up of the structure. Such information relating to the quality of the material can be obtained, for example, by measuring the propagation characteristic of the ultrasonic wave or the scattering property within the structure. In the case of measuring the ultrasonic wave propagation characteristic or the ultrasonic wave scattering characteristic in accordance with the pulse reflection method, as will be described hereinafter, difficulty would be encountered to independently measure one or both the characteristics. The ultrasonic wave propagation characteristic, particularly the ultrasonic attenuation characteristic, and the ultrasonic scattering characteristic are in closed relation to each other as described in "Journal of Statistical Physics", Vol. 36, Nos. 516, Pages 779 to 786 published in 1984. On the other hand, if there is the possibility to independently obtain the ultrasonic propagation characteristic and the ultrasonic scattering characteristic, the possibility may result in quantitatively understanding the quality of the material, particularly polycrystal portion of a metal and the like or the quality of the defect portion or the like within a sintered body. The conventional method of measuring the ultrasonic propagation characteristic will be described hereinbelow with reference to FIG. 1 for a better understanding.
In FIG. 1, numeral 101 represents a body to be examined and numeral 102 designates an ultrasonic transducer for transmitting an ultrasonic wave into the body 101 and receiving the ultrasonic wave reflected from the inside of the body 101. Between the body 101 and the ultrasonic transducer 102 is provided a coupling medium 103 for performing an acoustic coupling therebetween which is encased in a container 104. The ultrasonic transducer 102 is driven by a pulse driver 105 and also coupled to a receiving section (preamplifier) 106 for amplifying the signal received thereby which is in turn coupled to a detector 107 for demodulating the output of the receiving section 106. Numeral 108 is a scan transducing section for storing the output of the detector 107 so as to form a cross-sectional image, followed by an indication section 109 for indicating the output of the scan transducing section 108. The receiving section 106 is also coupled to a signal analysis section 110 for performing the frequency analysis with respect to the output of the receiving section 106, the output of which is supplied to the scan transducing section 108 and indicated at the indication section 109.
In operation of the apparatus thus arranged, the pulse driver 105 initially generates a drive pulse signal which is supplied to the ultrasonic transducer 102 to produce an ultrasonic pulse. The ultrasonic pulse from the ultrasonic transducer passes through the coupling medium 103 and reaches the body 101. The coupling medium 103 may be composed of a material such as water whose ultrasonic absorption is small. The container 104 is for the purpose of preventing the flowing-out of the coupling medium 103. Here, the body 101 to be examined is a portion of a structure made up of a casting and contains a polycrystal portion of uneven quality. A portion of the ultrasonic pulse reaching the body 101 is propagated thereinto and scattered successively in response to variation of the acoustic quality therein. A portion of the scattered ultrasonic pulse goes reversely to the propagating path, i.e., the acoustic scan line, and returns back to the ultrasonic transducer 102 where it is converted into a reception signal. In steps of the propagation and the scattering, the ultrasonic pulse is affected by the acoustic quality of the body 101, i.e., the ultrasonic wave propagation characteristic and the ultrasonic wave scattering characteristic thereof. The reception signal is amplified in the receiving section 106 the output of which is detected by the detector 107 whose output is in turn stored and scan-transduced in the scan transducing section 108 the output of which is thus indicated at the indication section 109 may comprising a standard TV monitor or the like. On the other hand, the output of the receiving section 106 is processed for a signal analysis such as frequency analysis in the signal analysis section 110 so as to obtain the propagation characteristic and the scattering characteristic. The attenuation characteristic being one of the propagation characteristic may be obtained as follows. That is, initially derived are the reception signal h(R1) corresponding to a predetermined depth R1 within the body 101 and the reception signal h(R2) corresponding to a predetermined depth R2 therein. Here, the length of the derived signal is 5 mm within the body 101, for example. When the length of the derived signal is estimated to be l mm and the corresponding gate width is T microseconds, the relation can be expressed in accordance with the following relational equation. ##EQU1## Thus, as understood from this equation, it is required that the acoustic velocity V has been already known to determine the gate width. Data representing the acoustic velocity is required to be prepared in advance in accordance with materials. Secondly, a frequency analysis such as Fourier transformation is effected with respect to the reception signals h(R1) and h(R2). When the Fourier transformation of the reception signal h(R1) results in H1(.omega.) and the Fourier transformation of the reception signal h(R2) causes H2(.omega.) where .omega. represents an angular frequency, H1(.omega.) and H2(.omega.) can be expressed as follows. EQU H1(.omega.)=T(.omega.).multidot.G1(.omega.).multidot.S1(.omega.) (3) EQU H2(.omega.)=T(.omega.).multidot.G2(.omega.).multidot.S2(.omega.) (4)
where T(.omega.) represents the frequency characteristic of an ultrasonic wave pulse transmitted and received by the ultrasonic transducer 102, G1(.omega.) and G2(.omega.) respectively represents propagation characteristics in which the ultrasonic wave receives during the movement-back-and-forth thereof between the ultrasonic transducer 102 and the positions of the depths R1 and R2, and S1(.omega.) and S2(.omega.) are respectively scattering characteristics of the ultrasonic wave at the depths R1 and R2.
As obvious from these equations, the reception signal includes the ultrasonic propagation characteristic and the ultrasonic scattering characteristic in state of multiplication whereby difficulty is encountered to independently obtain them. However, if the scattering characteristics S1(.omega.) and S2(.omega.) are equal to each other, the propagation characteristic G21(.omega.) between the depths R1 and R2 within the body 101 can be obtained by taking the ratio of the equations (3) and (4). That is, ##EQU2## Here, the absolute value of the propagation characteristic G21(.omega.) corresponds to the ultrasonic attenuation characteristic under the condition of no diffraction of the ultrasonic beam. Thus, the propagation characteristic of the ultrasonic wave can be obtained for predetermined regions within the body 101 to be examined, and the propagation characteristic G1(.omega.) between a surface of the body 101 and the position of the depth R1 can be also obtained and further the scattering characteristic S1(.omega.) can be obtained in accordance with the equation (3). However, in the case that the body 101 to be examined is composed of a scattering body of uneven quality, the ultrasonic scattering characteristic depends largely on the place and therefore it is impossible to estimate that S1(.omega.) and S2(.omega.) are equal to each other. On the other hand, contrary to this, utilized reversely is the fact that the ultrasonic scattering characteristic is varied at random in accordance with variation of the place to be examined. That is, the ultrasonic transducer 102 is moved in a predetermined range in directions parallel to the surface of the body 101 to obtain a number of reception signals from a number of places within the body 101, each of which is processed by the frequency analysis, and the results of the frequency analysis are averaged to cancel only the scattering characteristics varied at random. With respect to the averaged Fourier-transformation results H1(.omega.) and H2(.omega.), the equation (4) can be applied, thus allowing obtaining the propagation characteristic and so on.
In such an arrangement, the cancelling of the scattering characteristics can be made under the condition that the scattering characteristic of the ultrasonic wave within the body 101 is varied significantly at random when the place to be examined is changed, that is, the places to be examined are not in correlation to each other at all. However, a problem encountered in such an arrangement is that this condition cannot be satisfied when there includes boundary surfaces having a precise acoustic boundary, resulting in not allowing the cancelling thereof in this case. In addition, another problem in such an arrangement is to lengthen the measuring time period because of many times of transmissions and receptions of the ultrasonic wave.